This invention relates to the preparation of oxyacids of phosphorus and esters thereof, and more particularly to novel and improved processes for the preparation and recovery of oxyacids, especially phosphorous acid.
Phosphorus-containing compounds such as oxyacids of phosphorus, organophosphates and organophosphites are important precursors for the synthesis of other phosphorus species, which have various applications, for example, in herbicides, insecticides, fertilizers, flame retardants and plasticizers. The syntheses of such oxyacids of phosphorus, organophosphates and organophosphites have commonly used a halogen derivative of phosphorus, such as PCl.sub.3 or POCl.sub.3, as a starting material. Nevertheless, since the halogen derivatives are themselves prepared from elemental phosphorus, an economic advantage could be gained by preparing oxyacids of phosphorus, organophosphates and organophosphites directly from elemental phosphorus. Such direct preparation could also provide environmental benefits by avoiding the use of halogen-containing phosphorus starting materials and production of halogen-containing byproducts.
White phosphorus, the elemental phosphorus allotrope also referred to as yellow phosphorus or tetraphosphorus (P.sub.4), is a potential starting point for the synthesis of a variety of phosphorus species. The tetrahedral structure of white phosphorus contains six phosphorus-phosphorus bonds and can provide a large number of reactive species having an intermediate existence in phosphorus reactions. The competition of these reactive species for the organic reactants, however, can give complex reaction mixtures with low yields of desired products.
Various references have disclosed processes for the aerobic oxidation of white phosphorus in the presence of alcohols, but the reported processes have not provided an economical procedure for obtaining desirable phosphorus-containing compounds with high yield and purity. For example, Ernsberger et al. (U.S. Pat. No. 2,661,364) describes a process for preparing dialkylphosphite wherein oxygen is introduced into a mixture of small cut-up pieces of white phosphorus and a monomeric, saturated alcohol exemplified by ethanol, n-butanol, n-propanol, isopropanol and n-hexanol. Ernsberger et al. indicates that the various reaction products are isolated from the reaction mixture by conventional techniques, e.g., removal of phosphorous acid by aqueous extraction and recovery of alcohol by distillation. Ernsberger et al. further state that some dialkylphosphites are too unstable to be separated by distillation and may be used without isolation from the reaction mixture.
Kellerman et al. (British Patent No. 1,112,976) describes a process for producing organophosphorus compounds by reaction of white phosphorus, oxygen and a hydrocarbon compound containing at least one phenolic hydroxy group, a hydrocarbon compound containing more than one alcoholic hydroxy group, or a hydrocarbon containing a thiol group, to produce a mixture of esters of oxyacids of phosphorus in which the overall oxidation state of phosphorus is substantially 4. The Kellerman et al. process may take place in an inert organic solvent. The product of the Kellerman et al. process is a viscous mixture of non-isolated esters of oxyacids of phosphorus.
Okamoto et al., Yukagaku 19(10):968-72 (1970) (translated) describes a process for preparing diethylphosphite by reacting white phosphorus, oxygen and ethanol, after which excess ethanol is removed by vacuum distillation to obtain a 43% yield of diethylphosphite. The reaction was also performed in water and carbon tetrachloride.
A need has remained in the art for improved processes for the preparation of oxyacids, and in particular for improved methods of separating mixtures that contain phosphorus oxyacids, organophosphites and/or organophosphates. There is a further need in the art for a process that directly converts phosphorus, most preferably white phosphorus, to oxyacids of phosphorus, organophosphates and/or organophosphites, wherein the various desired products are obtained in high yield and purity and with economical isolation techniques. There is also a need for such a process that is environmentally safe, preferably by using low levels of solvents and avoiding the use of halogen-containing starting materials.